T53£0 



Case 
Carbonizing 



Case 
Carbonizing 



DRIVER-HARRIS COMPANY 

Harrison, N. J. 






$° 



Copyright, 1920 

by 

Driver-Harris Company 



aa-ffltl 



Essex Press, Inc. 
Printers and Binders 



NOV ~2 1920 Newark, N.J. 

©C1A599940 



INTRODUCTION 

The rapid growth of the steel industry in the past thirty 
years has been reflected along all lines of metallurgical work. 

The progress has been stimulated at every step by the 
scientific investigation of the problems involved. This scientific 
control has resulted in introducing economies in the operation 
of the older processes and in devising new methods for the at- 
tainment of new ends. 

It is not surprising, then, to find that, in the field of "Case 
Carbonizing" or "Case Hardening," a large amount of infor- 
mation has been accumulated which is of value to the heat 
treater. It is no longer necessary to carry out the process of 
"Case Hardening" along purely empirical lines. The chemical 
reactions which are involved in the various methods used for 
"Case Hardening" are now well understood. The quality of 
the steel which should be used, the nature of the carbonizing 
compound, the temperature and time of the operation and the 
nature of the container have all been subject to investigation. 

While this information is to be found in the scientific files, 
it frequently happens that it is not available to the heat treaters 
on whom devolves the practical application of the heat treating 
processes involved. 

This booklet, then, aims to lay before the heat treater, in 
language which can be readily understood, the information 
which has been rendered available by the more recent scientific 
investigations. 



Table of Contents 



CHAPTER I. 

Case Carbonizing 

Definition — History — Mechanics — Oil Tempering vs. 
Case Hardening — Requirements for Case Hardening — Qual- 
ity of Steels Used — Effect of Temperature and Time — Pack 
Hardening — Carbonizing Compounds — Containers — Packing 
— Soft Spots — Vertical Pack Hardening. 

CHAPTER II. 
Cyanide Hardening 

CHAPTER III. 
Gas Hardening 

CHAPTER IV. 

Lead Tempering and Hardening 

CHAPTER V. 
Carbonizing Containers 

CHAPTER VI. 

Nichrome — Commercial and Technical Data 

CHAPTER VII. 

Cast Nichrome Containers 

Pack Hardening — Cyanide Hardening — Lead Hardening 
— Pyrometer Protection Tubes — Other Applications. 

CHAPTER VIII. 
Commercial Methods for Using Nichrome Castings 

APPENDIX 
Stock Patterns and Special Containers 



CHAPTER I 



CASE CARBONIZING 



Definition — History — Mechanics — Oil Tempering vs. Case 
Hardening — Requirements — Steels — Temperature and Time — 
Pack Hardening — Carbonizing Compounds — Containers — 
Packing — Soft Spots — Vertical Pack Hardening. 



CHAPTER I. 

Case Carbonizing 

Definition: 

The affinity of iron, and likewise of steel, for carbon is 
so great that if a piece be heated at a bright red heat for 
some time, in contact with some material capable of giving up 
carbon, the iron or steel will readily absorb carbon. If this 
heating be prolonged over a period of several days, and the 
amount of carbon absorbed be considerable, then the opera- 
tion is known as "Cementation." If, however, the operation 
be of comparatively short duration, say over a period of a few 
hours, then the operation is known as "Carbonizing" or "Case 
Hardening." We may therefore say that Carbonizing or Case 
Hardening is the operation of heating iron or steel, in intimate 
contact with carbonaceous material, at a temperature above the 
critical point of the particular steel under treatment, for a short 
period of time. 

In foreign countries the word cementation is used almost 
entirely when discussing the carbonizing of steel. In the 
United States the word "Cementation" has come, through 
common usage, to mean the process of making steel by adding 
carbon to wrought iron through an absorption operation. For 
the sake of clearness the words "carbonizing" and "case harden- 
ing" will be used throughout this treatise. 

As the name "Case-hardening" implies, there are two sepa- 
rate and distinct operations to the process. The first operation 
produces the case, and the second operation consists of the 
proper heat treatment of the piece after the case has been 



8 Driver-Harris Company 

secured. Strictly speaking, therefore, the process commonly 
known as case-hardening really consists of a case-carbonizing 
operation and a case-hardening operation. 

History: 

The process of case-hardening was known to workers in 
ferrous metals many centuries ago. To state just how ancient 
the process is, is to go beyond the point of present human 
knowledge. We do know, however, that the Chinese were 
familiar with case-hardening as early as the eighth century. It 
is also known that in the ninth century, a Benedictine Monk 
wrote a book on case-hardening in which explicit instructions 
were given as to the correct method for case hardening files. 
It appears that files and ragged tooth saws were the first imple- 
ments to be case-hardened. It is interesting to note that the 
earliest case-hardening of which we have record was done by 
wrapping the piece to be hardened in a piece of skin which was 
then burned. This was the forerunner of the charred leather 
in use to some extent today. The book of the ninth century 
went so far as to give a formula for the carbonizing compound 
in which the hardening was to be done, and which consisted of 
three parts of charred horn to one part of salt. 

Since these earliest experiments the process of case-harden- 
ing has, of course, undergone many radical changes. The 
progress was slow during the middle ages, but it has been espe- 
cially rapid in the last thirty-five years. 

Mechanics : 

As previously stated, case-hardening is the operation of 
heating iron or steel in contact with carbonaceous material for 
a short period of time at a bright red heat. To the practical 
heat treater, case-carbonizing has come to mean the production 
of a high carbon surface on a piece of low carbon steel. In 



Case Carbonizing 



other words, case-carbonizing means the introduction of solid 
carbon into the surface of iron or steel, or the absorption of 
carbon by the surface. The high carbon content of the surface 
is made possible by the mechanics of the carbonizing process. 
Case-carbonizing is entirely dependent on the fundamental fact 
that iron or steel will readily absorb carbon at temperatures 
above the critical point of that particular iron or steel. The 
correct heat treatment, after case carbonization, gives the nec- 
essary hardness to the surface and leaves the core with a requi- 
site softness and toughness, thus making the piece case hard- 
ened. 

The object of case-hardening can therefore be stated to 
be: "To produce a piece of steel having the requisite surface 
hardness to withstand wear and abrasion, and having a core or 
center, soft and tough to withstand sudden or continued shock." 

It is well known that a steel low in carbon is soft, tough, 
malleable and ductile. A steel of this nature will not with- 
stand wear or abrasion. Conversely, a steel high in carbon is 
neither soft nor tough, nor is it malleable or ductile; but such 
a steel will withstand both wear and abrasion. For the moving 
parts of machines, the combination of the properties of a low 
carbon steel with the properties of a high carbon steel, should 
therefore produce a steel well able to withstand wear, abrasion, 
shock and impact. To secure such a result we must turn to 
case-hardening, for there we have the high carbon surface to 
withstand wear and abrasion, and the low carbon core or center 
to withstand shock, impact and fatigue. In case-hardened steel 
we have a high carbon surface capable of being heat treated to 
produce a maximum surface hardness, and a low carbon core 
which is soft, ductile and tough. 

The chemical reaction by means of which case-hardening 
is brought about cannot be readily explained without entering 
into a detailed technical explanation, which is outside the scope 



io Driver-Harris Company 

of this discussion. It can, therefore, be only briefly touched 
upon. 

Steel, when sufficiently heated, readily absorbs gases. But 
all gases are not absorbed with equal speed, or in equal quan- 
tities. Oxygen, nitrogen, hydrogen, and gases containing car- 
bon are all absorbed, but in varying quantities. Oxygen and 
hydrogen are absorbed very sparingly, and the steel seems to 
become saturated very quickly. With gases containing carbon, 
however, this is not the case. Large quantities of these may 
be absorbed by the steel by reason of the great affinity of iron 
and steel for the carbon which is contained in these gases. This 
fact will be entered into more thoroughly in one of the follow- 
ing chapters devoted to "Gas Hardening." Suffice it to say 
here, that the mere fact that steel will absorb carbon gases un- 
der certain definite conditions is the reason why case-hardening 
is possible, since it will be remembered that case-hardening is 
an introduction of carbon into the surface. 

Oil Tempering vs. Case-Hardening : 

There has been much discussion from time to time as to 
the relative advantages of oil tempering and case-hardening. It 
might be well, therefore, to discuss the advantages and disad- 
vantages of the two processes for achieving similar results. 

Can oil tempering give the same final results that can be 
obtained by case-hardening? In order to answer intelligently, 
it must be understood very clearly that there are important 
differences between an oil-tempering steel and a case-harden- 
ing steel. We have already seen that a case-hardening steel has 
properties, after heat treating, that are inherent in both low 
carbon steels and high carbon steels. Consequently, case hard- 
ening gives the beneficial effects of both steels. 

All metallurgists and heat treaters are familiar with the 
fact that a low carbon steel cannot be oil-tempered. There- 



Case Ca rbonizing i i 

fore, if oil-tempered pieces are to be produced a rather high 
carbon steel must be used. On the other hand, it is also a fact 
that a high carbon steel, after hardening, cannot possibly have 
a soft or tough core. The comparisons of the two methods of 
securing a hard wearing surface center around those two basic 
facts. By the process of case-hardening we obtain the hard 
surface to withstand wear and abrasion, together with the soft 
center to withstand shock and fatigue. In oil-tempered stock 
it is not possible to have a soft core and a hard surface, so we 
must necessarily come to the conclusion that the use to which 
the part is to be put must ultimately determine whether case- 
hardened stock or oil-tempered stock is to be used. 

Many laboratories have devoted much effort along dif- 
ferent lines to determine the answer to this problem, and most 
of this investigation work has been done on gears. The sub- 
ject of "Gear Hardening" will be taken up in its proper place 
in a later chapter. 

Requirements : 

Having discussed the fundamentals of case-hardening or 
carbonizing, we may now proceed with the discussion of the 
principles on which the results of the case-hardening process 
depend. 

In general, it can be said that the case-hardening process 
depends on five important factors: 

i. Chemical analysis of steel to be case-hardened. 

2. Temperature of operation. 

3. Length of time consumed by operation. 

4. Type of carbonizing material used. 

5. Type of container used. 

While the process itself is dependent on the five factors 
enumerated above, the results to be obtained from the case- 



12 Driver-Harris Company 

hardening operation depend on still another factor, namely: 
The heat treatment to which the case-hardened piece is subse- 
quently subjected. 

A steel high in carbon will resist wear to a greater extent 
than a low carbon steel. But since a steel high in carbon is 
capable of being made still harder by the proper heat treatment, 
it would obviously be unwise to stop after putting the case on 
the piece to be hardened, when a much harder surface can be 
obtained by the proper heat treatment. This, then, leads us to 
the discussion of the steels most often used for case-hardening. 



Steels : 

The steels most commonly used for case-hardening are 
low carbon steels with a carbon content of .16 to .22. The 
impression must not be gathered, however, that only carbon 
steels are used for this work. Many of the alloy steels are 
excellent case-hardening steels, and these will be discussed in 
the proper place. Generally speaking, it can be stated here 
that manganese, chromium, tungsten, and molybdenum are all 
elements which, when alloyed with steel, increase the rate of 
carbonization, while silicon, aluminum, nickel and titanium, 
decrease the rate. 

Since carbon steels are the class of steels most generally 
case-hardened, they will be discussed first. When the carbon 
content in steels is more than .25% it tends to increase the 
brittleness of the steel. The success of case-hardening depends 
on the fact that it is possible to maintain a very soft core which 
will also be tough. Therefore, it would be decidedly unwise to 
start with a steel that has an initially high carbon content, be- 
cause the start would then be made with a steel having an ini- 
tial tendency toward brittleness. It must be remembered, 
however, that steels containing as much as .55% carbon are 



Case Carbonizing 13 



used for case-hardening where stiffness and rigidity in the core 
are more important than softness or toughness. 

It might be asked here why steels with carbon contents 
under the low limit mentioned above are not used, since soft- 
ness and toughness would be increased. Some foreign manu- 
facturers do use and recommend steels for case-hardening with 
a carbon content as low as .10. Steels with such low carbon 
content, however, are more fibrous than crystalline, so that 
they do not machine easily, and more grinding is necessary 
than profitable operation will allow. For these reasons the 
American heat treater has standardized on a .16 to .20 carbon 
steel. It must always be remembered that the higher the ini- 
tial carbon in the steel, the greater will be the concentration or 
percentage of carbon in the case. In this respect the initial 
carbon is quite important. 

The manganese content of a case-hardening steel should 
not be high, because manganese acts in two ways which are 
not beneficial. Manganese lowers the critical point of the 
steel, thereby increasing the degree of overheating that the 
steel will get during the case-hardening operation. Mangan- 
ese also increases the brittleness of the case. Good American 
practice calls for a manganese content under .35%. Although 
it has been stated that the presence of any appreciable amount 
of manganese lowers the critical point and increases brittleness, 
it must not be assumed that a steel with a higher percentage of 
manganese than .35 is never used for case-hardening. It is 
sometimes necessary to use a steel which will give the finished 
product great stiffness. For this purpose a steel having a man- 
ganese content of from .75 to .85 is sometimes used. This is 
especially true of many products made by English manufac- 
turers and particularly of manufacturers in the Sheffield Dis- 
trict. 



Driver-Harris Company 



The silicon content of a case-hardening steel should gen- 
erally be under .25, for the reason that silicon decreases the 
speed of carbonization and also its depth of penetration. 

The phosphorus and sulfur, of course, should be as low 
as possible, because a case-hardened steel should be exception- 
ally free from impurities and segregation. 

Among the alloy steels, chromium and nickel steels are 
excellent for case-hardening. Chromium makes the case ex- 
tremely hard and raises the critical point, thereby raising the 
temperature at which overheating can occur. Chromium also 
makes the grain finer and the steel reacts more readily to heat 
treatment. Chromium up to 1.50% is frequently used in case- 
hardened steels, but in percentages less than .50 does not seem 
to have much effect. 

Nickel steel is a very good case-hardening steel for several 
reasons. A nickel steel having a nickel content of from 2.00 
to 3-5% is excellent for parts that are important enough to 
carry the added initial cost of the nickel steel over a carbon 
steel. Nickel increases the strength of steel, and makes the case 
less liable to crack by reducing its brittleness. Nickel does not 
make the steel as hard to machine as does Chromium. 

Vanadium, as we would expect from our knowledge of 
the action of vanadium when alloyed with steel, increases its 
resistance to fatigue by increasing the toughness of the core. 



Temperature and Time: 

The next important consideration is the temperature at 
which the case-hardening is done. On the temperature of 
operation depends, to a large extent, the depth of penetration; 
also the percentage or concentration of carbon in the case, and 
the structure of the core after the operation is completed. 



Case Carbonizing 15 



Since temperature of operation and time of exposure to 
carbonizing influences are interdependent, it will be well to 
discuss them together. Correct temperatures without a suffi- 
ciently long exposure would be as ineffective as long exposure 
without sufficient temperature. 

Case-hardening may be accomplished in any one of three 
ways, that is, by pack-hardening, liquid-hardening or gas-hard- 
ening. We will first make it clear as to what is meant by pack- 
hardening before proceeding with the discussion of tempera- 
ture and time elements. 

Pack Hardening: 

By "pack-hardening" is meant "the operation of putting a 
high carbon case on a piece of soft steel by packing the steel in 
a suitable closed container in intimate contact with some solid 
carbonaceous material, and submitting the whole to a suitable 
temperature for a short period of time." 

The absorption of carbon by a piece of steel during the 
pack-hardening operation is accomplished by raising the tem- 
perature above the critical point of the steel that is being 
treated. Below the critical point the steel absorbs little, if any, 
carbon. This is true even though the time of the operation be 
extended over long periods. Above the critical point, however, 
the steel has a great affinity for carbon, and the depth of the 
penetration is greatly increased as the temperature rises higher 
and higher above the critical point. 

It is natural to inquire whether, by increasing the tem- 
perature beyond the ordinary temperature at which the opera- 
tion is done, the time of the operation could not be greatly cut 
down. The interdependence of temperature and time is one 
answer to this question, because the higher the temperature the 
greater the amount of over-heating will be, and although the 
time be greatly shortened, the bad effects of over-heating will 



_i6 Driver-Harris Company 

be present nevertheless, since an excessive temperature coupled 
with a short time exposure will produce as bad results as a 
longer exposure at a lower temperature. This bad effect of 
high temperature and long exposure is known to heat treaters 
to produce a coarse grain in the core, and as a result the ma- 
terial loses its softness and toughness and gains in brittleness. 
The second answer to the question is that carbonizing proceeds 
much more rapidly between 1600 ' F. and 1650 F. than it 
does above 1650 F. There are special cases where it is neces- 
sary to carbonize at temperatures as high as 1850 F. but such 
temperatures are the exception and not the rule. It can be 
taken as standard American practice to case-harden at a tem- 
perature of from 1600 F. to 1700 F. 

Case-hardening, as stated before, proceeds very slowly, if 
at all, at temperatures under the critical point of the steel. If 
it does proceed, it is only a very superficial or surface case. It 
is therefore clear that if the depth of the case must be very 
great, a high temperature is absolutely necessary, because a low 
temperature and long exposure will not produce the desired 
penetration. 

The length of time for which the steel is exposed to car- 
bonizing influences is also important. This is seen from the 
fact that the time of exposure under constant temperatures is 
the factor which regulates the depth of the penetration of the 
case. It follows, then, that by securing the proper depth of 
penetration, the percentage or concentration of carbon on the 
surface will also be correct. 

One of the fundamentals of case-hardening is that the 
carbon content of the case varies from the surface inwardly to 
the core. The surface will have the greatest carbon content, 
and this gradually becomes less as the case is turned off and the 
core is reached. The lower the temperature, the shorter the 
time of the operation, and the lower the initial carbon in the 



Case Carbonizing 17 



steel, the greater will be this reduction in carbon content from 
surface to core. 

As the carbonizing process proceeds, that is, as the steel 
becomes more fully carbonized, additional carbonization re- 
quires longer and longer time of exposure. In ordinary case- 
hardening processes the average rate of penetration is about .80 
mm. or .0315 inch per hour. 

The production of a case containing more than .9% car- 
bon is never advisable, because cases with more than .9% 
carbon are liable to crack and spall. This is true for all ordi- 
nary work. The production of a very deep case, without caus- 
ing the concentration of carbon at the surface to be over .9 to 
1.0%, can be accomplished by long treatment or exposure at a 
temperature around 1 575° F. This is true notwithstanding 
the general fact that the concentration of carbon on the sur- 
face increases with the depth of the case. If the temperature 
be high enough, and the time of exposure be sufficiently long, 
it is easy to carbonize small parts all the way through their 
diameter, thus offsetting any benefits which the successful case- 
hardening operation gives. 

Substantiating what has gone before, it can be said that 
the practical heat treater is gradually coming to the viewpoint 
that low temperature carbonizing gives as efficient results as 
carbonizing at high temperatures, and that the added time 
necessary to carbonize at low temperatures is more than offset 
by the saving in fuel, furnaces, containers, carbonizing com- 
pound, and life of the finished article. In addition, the final 
heat treatment is simpler to perform if the carbonizing has been 
done at a lower temperature. These elements of cost also tell 
the heat treater that it is unwise to put a deeper case on the 
steel than its ultimate use warrants. The cost of straightening 
case-hardened pieces which have become warped by high 



1 8 Driver-Harris Company 

temperatures is another factor in favor of low temperature 
carbonization. 

If there is any exception to the low temperature principle 
it is only because of very special requirements. For example: 
It is the practice of one industrial plant to change or oscillate 
the temperature every two hours during the case-hardening 
operation. This plant wishes to obtain a surface hardness 
caused by "cementite" forming in the steel, and brittleness in 
the finished product is a minor consideration. Generally speak- 
ing, however, the lower and uniformly regulated temperature 
gives the best results. 

Carbonizing Compounds: 

It may be true that choice of carbonizing compound is 
not so important as are time and temperature. It is a mistake, 
however, to consider the nature of the compound as unimpor- 
tant, and the heat treater who is interested in saving time and 
keeping down costs should be careful to use compounds con- 
taining the proper qualities for carbonizing. 

A carbonizing compound is effective only as long as it 
will release or give up free carbon in one form or another. 
The term free carbon does not necessarily mean free solid car- 
bon, but rather some gas capable of giving up free carbon upon 
dissociation. It has been proven that pure, free carbon is not 
a satisfactory substance for carbonizing, because the length of 
exposure necessary and the temperature that must be main- 
tained make it unsuitable for present-day manufacturing meth- 
ods. It can be stated then, that while pure carbon in contact 
with steel will ultimately carbonize the steel, its use is im- 
practical. 

Since it is true that some gas capable of giving up free 
carbon can be used for carbonizing, we naturally come to the 
discussion of compounds which will liberate gases containing 



Case Carbonizing 19 



carbon. Most compounds that can be depended on for car- 
bonizing are mixtures of some of the following substances, two 
or more of which are contained in most commercial grades on 
the market: wood charcoal, ground bone, anthracite coal, 
ground coke, charred leather, horn, animal black, lamp black, 
graphite, barium carbonate, sodium chloride (common salt), 
barium chloride, potassium or sodium cyanide. Ready mixed 
carbonizing compounds of varying content are sold by manu- 
facturers, so that the heat treater need merely determine which 
compound meets his particular requirements. Generally speak- 
ing, the amount and richness of the carbon gases which they 
are able to produce should govern the choice of the compound. 
All carbonizing compounds, whether granule, flake, or 
powder, must necessarily have atmospheric air occluded in 
them when they are packed in a container. Air, of course, is 
an intimate mixture of oxygen and nitrogen. When the tem- 
perature has risen so that action of the free oxygen of the 
occluded air is possible with the carbon in the compound, carbon 
dioxide is formed. As the temperature in the carbonizing box 
rises and approaches the carbonizing temperature, the carbon 
dioxide gas comes in contact with the red hot carbon in the 
compound and is dissociated into carbon monoxide, which is 
the active carbonizing agent. These actions can be clearly ex- 
pressed in the following two equations:* 

C + O v = C0 2 

Carbon + Oxygen = Carbon dioxide 

C0 2 + C = 2 CO 

Carbon dioxide + Carbon = Carbon monoxide 

The carbon monoxide then comes in contact with the steel 
which is packed in the carbonizing compound, with the result 
that the carbon monoxide is broken up, giving some of its car- 
bon to the steel, which readily absorbs it and thereby becomes 

*Note: — If the reader wishes to study these phenomena in detail 
he should consult Schenck "Physical Chemistry of the Metals." Chapt. 
IV. & V. 



20 Driver-Harris Company 

carbonized. This reaction may be represented by the follow- 
ing equation: 

2 CO = C0 2 + C 

Carbon monoxide = Carbon dioxide + Carbon 

The above, then, is what actually happens in the case- 
hardening process during the actual carbonizing operation. 
While practically any carbonizing compound will give the re- 
sults just mentioned, it will do so only as long as there is free 
carbon remaining in the compound on which the carbon dioxide 
gas can act. The compound chosen should therefore contain 
a large percentage of carbon. For this reason, the same com- 
pound should not be used over and over again without being 
replenished with new material. Good practice calls for a 25% 
addition of new carbonizing material to each container full of 
old material, before the old material is again used. 

It has been maintained for some time that compounds 
capable of liberating some free nitrogen are advantageous in 
case-carbonizing, but this is very doubtful. 

While salt is a part of many commercial carbonizing com- 
pounds, metallurgists are undecided as to its influence on the 
actual case-hardening operation. Many practical heat treaters 
swear by it as an active agent in increasing the speed of car- 
bonizing, but on the whole it may be said that the beneficial 
effect of salt is doubtful. 

Again, there has been a point of difference among metal- 
lurgists, for many years, as to the action of barium carbonate 
contained in some compounds. It has been maintained that 
cyanides were produced from these carbonates due to the action 
of the free nitrogen in the occluded air in the compound. It 
is more probable, however, that carbon monoxide is derived 
from the carbonate through the carbonate dissociating and 
giving off carbon dioxide, which later reacts with the hot car- 
bon to form carbon monoxide. The process of case-hardening 



Case Carbonizing 21 

through the use of carbon monoxide gas will be thoroughly 
discussed under the chapter on "Gas-Hardening." The fact 
that most compounds contain some bone or leather and that 
they therefore, upon decomposition, give off hydrocarbon gases, 
is another matter which will be discussed under "Gas-Harden- 
ing." 

There is one more consideration worthy of mention in 
connection with the choosing of a compound. Some com- 
pounds are capable of carbonizing gradually at low tempera- 
tures; that is, at temperatures very little above the critical 
point, because they give up their carbon gases slowly. These 
same compounds, at high temperatures, may give off gases en- 
tirely too rapidly, thereby greatly increasing the concentration 
of carbon on the surface of the case-hardened piece. However, 
compounds which act very quickly are generally used for super- 
ficial cases. 

Generally speaking, an impure form of carbon is better 
than pure free carbon for case-hardening, and the fewer the 
ingredients of the compound, the better the compound will be. 

Containers: 

Carbonizing boxes or containers, and their relation to effi- 
cient carbonizing in the broad sense, will be discussed thor- 
oughly in a later chapter. At this point we are interested only 
with the size and shape of the containers in relation to the 
work to be carbonized. 

When a packed box is placed into the furnace, the outside 
of the box and the pieces nearest the walls will naturally arrive 
at the furnace temperature before furnace temperature reaches 
the center of the box. This means that the pieces near the 
walls of the box may begin to carbonize before the pieces in 
the center of the box reach the critical temperature. The 
larger the box the greater will be this lag in both time and 



22 Driver-Harris Company 

temperature, and experience has shown that the lag in tempera- 
ture may be from two to four hundred degrees. This lag of 
temperature is always proportional to the size of the box and 
never can be changed by regulation of the furnace tempera- 
ture. It is therefore obvious that the box or container should 
be as small as is possible for manufacturing economy. 

Packing: 

It is difficult to discuss or lay down any fundamental rules 
for packing the pieces in the box. Most heat treaters have 
methods for their particular work which they have learned 
from long experience. It can be said, however, that even dis- 
tribution of the packing material around the pieces is very 
essential. It is also vital that extreme care be taken to pack 
small or light parts in a box in such a way that they will not 
carbonize all the way through, thus spoiling the benefits of the 
case-hardening. If small boxes are not available for carboniz- 
ing small parts, then carbonizing compound which has been 
previously used should be used again, so that the parts which 
will be on the outside rows in the boxes will not carbonize 
before the inner rows have reached the carbonizing tempera- 
ture. Large, heavy parts should be packed in shallow boxes, so 
that the heat at which the operation is carried on cannot cause 
them to sag and distort. 

Observations and experiences gathered from many repre- 
sentative plants, show that a layer of compound from i^ to 
2J/2 inches thick, is generally put on the bottom of the box, no 
matter what kind of material is to be carbonized. Pieces to be 
carbonized are next put in, placed in even rows or staggered, 
or in any other suitable manner, care being taken that the 
pieces do not touch each other or the sides of the box. Good 
practice calls for at least one inch of carbonizing material be- 
tween the walls of the box and the row of pieces placed next to 



Case Carbonizing 23 

the wall. The depth of the case desired must determine this 
figure to a great extent, and must also determine the distance 
or space between each piece. 

After the pieces are arranged to the satisfaction of the 
heat treater, a covering of at least i*/£ inches of carbonizing 
material is put over the pieces before the next layer of pieces 
is packed in. As many layers of pieces on top of each other as 
the box size will warrant, can be packed in, if allowance is 
made for at least two inches of carbonizing material on top 
of the last layer. Some very careful heat treaters make a 
practice of using a three-inch blanket of carbonizing material 
as a cover over the top layer of pieces to be carbonized. 

After the box is carefully packed, a metal cover should be 
put on. It has been customary to make this cover tight by 
luting with fire clay. Containers are made today, however, 
which require no luting of this kind, and these will be de- 
scribed in the chapter devoted to a detailed discussion of con- 
tainers for Pack Hardening. 

Soft Spots: 

Low carbon spots in high carbon cases are known as "soft 
spots." These soft spots occur in carbonized pieces wherever 
the carbon fails to penetrate properly. 

Poor packing and poor arrangement of the pieces in the 
carbonizing box cause soft spots so often that too much care 
cannot be taken in placing the pieces into the boxes and in pack- 
ing them in the compound. The pieces should always be evenly 
surrounded with uniform quantities of the compound, as more 
carbon may be absorbed where the compound covering is thick 
and less carbon where the covering is thin. When pieces touch 
each other, soft spots are sure to occur at the points of contact. 
Foreign substances in the compound, such as clay, sand or other 



24 Driver-Harris Company 

solid materials of an inert nature, will cause soft spots if they 
come in contact with the pieces during the carbonizing process. 
If an appreciable quantity of sulphur is absorbed during the 
carbonizing, it tends to produce a soft case. 

Since soft spots are caused by failure of the carbon to pene- 
trate at certain spots, it is readily seen that uneven heating and 
loss of gases through cracks or holes should be guarded against 
with every possible means. 

Uneven heating has a tendency to concentrate the carbon 
in the case, as well as to make the case of varied depth. Loss 
of carbonizing gases through cracks or holes in the container is 
equivalent to uneven heating and is responsible for all kinds 
of soft spots and spoilage. 

It is not always easy to detect pin holes, and even cracks 
are not always readily seen in their first stages. It is also not 
easy to determine just to what extent cracking, warping or 
scaling interferes with heat penetration and carbonizing action. 
But close study has shown that these conditions are responsible, 
in very large measure, for the fact that the largest single item 
of cost in connection with pack-hardening is due to cracking, 
warping and scaling of the containers. 

Soft spots that are the result of poor packing can be elimi- 
nated only by proper packing and the foregoing serves as a good 
general guide. Soft spots, caused by loss of gases due to crack- 
ing, warping and scaling of containers, can also be reduced to 
a negligible point if advantage is taken of the progress that has 
been made in the development and manufacture of containers. 
A later chapter will deal very specifically with containers that 
are proof against cracking, warping and scaling to such a degree 
that they have proved themselves many times more efficient 
from every standpoint. 



Case Carbonizing 25 



Vertical Pack Hardening: 

Before leaving the subject of pack hardening, it is desir- 
able to discuss briefly a comparatively recent development 
known as Vertical Pack Hardening. This consists of a vertical 
retort surrounded by a brick combustion chamber, in which the 
parts to be carbonized are fed through the top along with the 
carbonizing compound. The parts remain in the retort with 
the compound for the requisite time at a specified temperature, 
and are withdrawn at the bottom by gravity. The speed of 
flow through the retort is controlled by a small trap at the 
bottom which allows the materials to be withdrawn at any 
desired speed. 



CHAPTER II. 

CYANIDE HARDENING 




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CHAPTER II 
Cyanide Hardening 

In the preceding chapter, mention was made of the fact 
that case-carbonizing could be accomplished by any one of three 
methods, dependent, to a great extent, on the use to which the 
finished article was to be put. In this chapter the second of 
the three methods will be discussed under the subject of liquid 
carbonizers, or specifically, "Cyanide Hardening." 

Perhaps the first attempt to case-carbonize by means of a 
liquid carbonizer, was made by immersing the steel to be car- 
bonized in a bath of molten cast iron. The difficulties of this 
method were soon evident, and its use was discontinued. 
However, as study and time were given to the subject of liquid 
carbonizers, some very good results were obtained, and case- 
carbonizing by means of a liquid carbonizer is now quite gen- 
erally employed. The development work made it very evident 
that high melting point solids must be dispensed with, and so 
salts that are capable of giving up carbon, and whose melting 
points are not excessive, were resorted to. Among these salts 
are the carbonates of sodium and potassium, and the cyanides 
of sodium and potassium. Other fusible salts have been used, 
but their use is of comparatively little importance. 

There are several marked differences, both in method and 
in results obtainable, between pack-hardening as described in 
the preceding chapter, and liquid carbonizing. In the pack- 
hardening operation the depth of case is directly dependent on 
the temperature and time. In the liquid carbonizing operation 
the case produced is superficial or "skin deep." Therefore, the 
depth of case desired must be one of the determining factors as 



30 Driver-Harris Company 

to whether liquid carbonizing can be successfully used for any 
particular job. A few thousandths of an inch is the extreme 
depth that good practice can secure from liquid carbonizers. In 
addition to the very superficial case obtained, it has been found 
that the carbon content does not fall off gradually from the 
case to the core, but that the difference in carbon percentage is 
quite marked. This means that the case will have the neces- 
sary carbon content for hardening, while just under the case 
the carbon will be very little higher than in the original steel. 
This being true, there must be some good reason why liquid 
carbonizing is used. The best reason is the uniformity of the 
case obtained and the speed with which it can be secured. An- 
other determining feature is the extreme surface hardness ob- 
tainable. Since the case is so very hard and the drop in the 
carbon content from the case to the core so sudden, it would 
be supposed that the case might be subject to brittleness 
and flaking. This is true, and steel should never be case- 
carbonized in a liquid carbonizer unless resistance to shock, 
fatigue, impact, etc., is not essential. So it is seen that by using 
a liquir carbonizer one of the salient features of a case-hard- 
ened steel is sacrificed, namely, its toughness and resistance to 
impact, shock, etc. The liquid method is excellent when these 
qualities are of secondary importance, and where extreme sur- 
face hardness is of primary importance. Extreme rigidity and 
maximum hardness are the real objects of this process. 

The salts most commonly used for liquid-carbonizing are, 
as stated before, the cyanides of potassium and sodium, which 
explains why the process has come to be known as the cyanide- 
hardening process. It must not be supposed, however, that 
only cyanides are used. Some users mix soda ash (sodium car- 
bonate) and common salt (sodium chloride) with the cyanide. 

Whatever salts are used, the theory of their action is 
essentially the same and depends on the release of a carbon gas 



Cyanide Hardening 31 

which will give up its carbon to the steel. The principle of the 
pack-hardening process and the liquid process are therefore the 
same. 

Since cyanides are the essential part of most liquid car- 
bonizers, the method of using it will be taken up in detail. 
Cyanide hardening can be accomplished in two ways. The 
piece to be hardened may be immersed in a bath of molten 
cyanide; or it may be sprinkled with a mixture of cyanide and 
some adhesive, then heated to the carbonizing temperature and 
the sprinkling and heating repeated until the desired depth of 
case is attained. This latter method is known as the "Cyanide 
Varnish Method." It has some applications, but is not used 
nearly so widely as the immersion method because it is not as 
efficient, nor so simple of operation. The immersion method is 
the one, then, that must be considered. 

For the immersion method, an open pot, either rectangular 
or round, is suspended in a furnace and filled with the salt 
which, when fused, forms the carbonizing bath. It is impossi- 
ble to give a standard formula for this salt because the heat 
treater generally uses one compounded as his experience dic- 
tates. Sodium or potassium cyanide alone will do the work but 
in almost every case some sodium chloride is added, and in 
some cases a further addition of sodium carbonate is made. One 
of the largest users of the "Cyanide-Hardening" process used a 
fused bath composed of 74% sodium cyanide and 26% sodium 
chloride. Another large user uses a bath of 33% sodium 
cyanide, 33% sodium chloride and 34% sodium carbonate. The 
only standard of the whole industry is that cyanide is invariably 
used. Extraordinary precautions must be taken to have the 
furnace fumes conducted outside the building, because the gases 
evolved from molten cyanide are disagreeable and extremely 
poisonous. 



32 Driver-Harris Company 

The temperature necessary for carbonizing by this method 
is very little over the critical point of the steel used, and car- 
bonizing is successfully completed at temperatures from 1450 
F. to 1600 F. although temperatures as high as 1750 F. are 
sometimes maintained. After the bath has reached the neces- 
sary temperature, the cold steel to be carbonized is immersed 
in it and kept in it until it is thoroughly heated to the tem- 
perature of the bath. This requires from six to fifteen minutes 
of time. If a greater depth of penetration than cyanide ordi- 
narily gives is desired, then the time of immersion should be 
longer, but longer immersion is not advisable because of the 
danger of creating non-uniform, high carbon spots which might 
spall and flake in use. Usually, quenching is done immediately 
after bringing the piece out of the bath, and the quenching is 
done in either water, lime water or oil. 

The distortion and warping is not excessive in this process 
if ordinary precautions are taken, such as rapid cooling and not 
too excessive temperatures or too long a time of immersion. 
The lag or elapsed time between withdrawal from the molten 
cyanide and quenching in the cooling liquid should be a mini- 
mum. 

Steel discs, washers, small screw parts, nuts, etc., are suc- 
cessfully hardened in cyanide. In general, parts of small di- 
mensions, or sections are hardened in cyanide because the depth 
of case can only be a minimum. There is seldom any danger 
of carbonizing small or thin pieces entirely through when the 
cyanide method is used. 

Molten cyanide is very injurious to most containers. Cast 
iron and steel last but a short time when used in connection 
with fused cyanides. Oxidation takes place readily, holes burn 
through the container and large cracks often appear. It is 
common for the life of an iron or steel container to be as short 
as fifty hours, although some users obtain as much as 150 hours 



Cyanide Hardening 33 

from their containers. But aside from the short life of the 
containers, the worst feature of their failure is that the molten 
cyanide leaks through the failed container into the furnace, 
where, if it is not noticed immediately, it ruins the fire brick 
lining and necessitates a shut down for repairs to the furnace. 
The consequence, of course, is a loss in production as well as 
an added cost of production. 

Cyanide containers that have overcome these difficulties in 
very large measure and have multiplied the efficiency many 
times will be described in a coming chapter. 



CHAPTER III. 

GAS HARDENING 




Gas Hardening Furnaces equipped with Rotary Retorts and Thermo 
Couple Protection Tubes of Cast Nichrorae. 



CHAPTER III 
Gas Hardening 

In this chapter will be discussed the third of the three 
methods of case-carbonizing. 

While it is a fact that in the strict sense of the term, any 
of the methods of carbonizing could be called "gas carboniz- 
ing," gas hardening is commonly used to describe the method 
of carbonizing in which a gas is employed directly, and in 
which the steel to be carbonized does not come in contact with 
a solid or liquid capable of giving up a carbonizing gas. It 
will be remembered that under the subject of "Pack-Harden- 
ing" in Chapter I, the statement was made that all case-car- 
bonizing was due to the action of carbonizing gases. It can 
readily be understood, therefore, why a free carbonizing gas, 
rather than a substance capable of liberating a carbon gas, 
should have some industrial applications. 

It might possibly be of benefit to again explain the reac- 
tions which take place during the carbonizing operation. 

The reactions involved are made possible by the oxygen 
present in the carbonizing compound or the carbonizing gas. 
The oxygen comes in contact with the carbon in the compound 
or gas and forms carbon dioxide, which, as the temperature 
increases, comes in contact with the red hot carbon and forms 
carbon monoxide which is the active carbonizing agent. The 
carbon monoxide then comes in contact with the steel and dis- 
sociates again into carbon dioxide and free carbon. The free 
carbon is absorbed by the steel, and carbonizes it. (See Chapter 
I. for the reactions.) 



38 Driver-Harris Company 



Since it is true that the carbon monoxide gas does the 
carbonizing, it is readily seen that any process which eliminates 
the reactions necessary for the production of the carbon mon- 
oxide during the operation, and supplies the gas directly, would 
greatly simplify the industrial process. 

While it has been said that carbon monoxide is the active 
carbonizing gas set free by most carbonizing compounds, it 
should further be understood that carbon monoxide, or any 
other gas, is active only in proportion to the carbon which it 
sets free. This carbon is set free and dissolved in the steel in 
two ways. 

Case i. A gas may dissociate upon contact with the steel 
and set free all the carbon which it can yield upon the surface 
of the steel. The carbon thus set free on the surface is gradu- 
ally absorbed by the steel and penetrates into the steel under 
the effect of temperature and time. Such a case is rarely met 
with in practice. It means that the gas evolved acts only as a 
carrier of the carbon and has no part in the actual carbonization. 

Case 2. The second manner of solution of the carbon in 
the steel is the one which is usually met with in industrial 
practice. The gas may dissociate in such a way that only a 
part of the carbon it carries is set free. As it penetrates further 
and further into the steel the gas sets free more and more carbon 
within the steel which absorbs it readily. In this instance, such 
variables as temperature, time of operation, pressure, etc., all 
have a direct bearing on the case produced. This second man- 
ner of gas carbonizing is the method of hardening by means of 
carbon monoxide. However, at temperatures below 1300 F., 
carbon monoxide may also act by liberating free carbon on the 
surface of the steel as mentioned in Case 1. But since such low 
temperatures are never used in industrial case carbonizing, it 
can be stated that carbon monoxide acts as explained under the 
second case above. 



Gas Hardening 39 



Carbon monoxide does carbonize, however, much more 
intensely if free carbon is present. 

The common industrial practice is to use compounds or 
gases, for carbonizing, which are capable of giving up carbon 
monoxide, but not all industrial processes depend exclusively 
on its use. Many applications using gaseous hydrocarbons are 
in use among the heat treaters, but the specific action of these 
hydrocarbon gases has been known only within recent times. 
It is almost certain that part of the carbonizing action of these 
gases is due to the deposition of free carbon on the surface of 
the steel as explained under Case I. But all the carbonization 
done by hydrocarbon gases is not due to this action. Some 
carbonization is also effected by the absorption of the gas by 
the steel. A mixture of carbon monoxide gas, with a small 
percentage of hydrocarbon gas, is an excellent carbonizing gas, 
and if the percentage of the hydrocarbon is low the action leaves 
the surface of the steel clean, while increasing the speed of the 
action. 

Mention has been made of the element of pressure in 
carbonizing. The culmination of all the hopes of the metal- 
lurgist has been the production of a process for successfully 
carbonizing under pressure. It has been shown by a number of 
experiments that the tendency for steel to carbonize is increased 
by an increase in the pressure under which the carbonizing 
takes place. But this is true only within certain limits, so that 
it is not feasible to carbonize at pressures greater than three 
atmospheres. While carbonizing is possible at greater pressures, 
it seems that the rapidity of the operation is greatest at about 
forty pounds pressure. Successful industrial applications of 
pressure carbonizing are still in their infancy, but at least one 
manufacturer of carbonizing equipment has some developments 
nearing completion for carbonizing by gas under pressure, and 



40 Driver-Harris Company 

the Driver-Harris Company is now developing a pressure pot 
for pack hardening under pressure. 

Industrial processes for carbonizing with gas consist, for 
the most part, of a rotary horizontal retort, rotating in a com- 
bustion chamber fitted with suitable burner equipment for se- 
curing the proper temperature. The pieces to be carbonized 
are fed into one end of the retort and are discharged at the 
opposite end after the required time has elapsed. Gas is forced 
into the retort and as the temperature is correct for carboniz- 
ing, the steel absorbs the carbon from the gas very readily. A 
detailed description of this furnace can be secured from the 
manufacturers. 

Gas carbonizing by this method is feasible for small parts 
such as bolts, nuts, threaded dies, small axles, bearings, chain 
links, buckles, etc. The advantages are that large numbers of 
small parts can be carbonized at one time and that the process 
is practically continuous. It is especially adapted to securing 
thin cases. Many large manufacturers of case carbonized parts 
are now using this process, and its field is practically unlimited. 



CHAPTER IV. 

LEAD HARDENING 




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CHAPTER IV 



Lead Hardening 

While strictly speaking, lead hardening is not a case-car- 
bonizing operation, the use of lead as a heat-treating medium 
for case-carbonized parts is extensive enough to make it worthy 
of mention here. 

It was stated in an earlier chapter that case-hardening 
consisted of two separate and distinct operations — the case-car- 
bonizing operation and the case-hardening operation. In other 
words, case-hardening consists of the production of the case 
and the hardening of the case after it has been produced. 

The decision as to whether oil, water, or lead should be 
used for the quenching bath rests to a great extent on the ex- 
perience of the heat treater and on the kind of work to be heat 
treated. No standard rules can be set down, and the use of 
any quenching bath must be ultimately determined by the par- 
ticular qualifications of that bath. Different manufacturers 
harden identical parts in entirely different quenching baths. 
Lead baths have been selected for discussion because the opera- 
tion involves a rather high temperature, and wherever high 
temperatures are used difficulties with containers at once fol- 
low. 

An installation for lead hardening consists of suitable 
container, supported in a suitable furnace equipped with a 
proper burner installation to give the required temperature. 
Containers may be of various shapes, dependent on the kind of 
work to be hardened. Containers are supplied round, rectan- 
gular or of bath tub design. The container is filled with the 



44 Driver-Harris Company 

lead which, when molten, forms the bath, and a covering of 
powdered or lump charcoal is added to prevent the surface of 
the lead from oxidizing. Sometimes a blanket of salt is used 
instead of the charcoal, but this is not to be recommended. 

The only reason for using a high temperature bath is that 
the object to be quenched or heated is surrounded by a uni- 
formly hot substance on all sides, and is not exposed to the 
oxidizing influence of the air. Due to the increased cost of 
heating or quenching by means of lead, its use is confined to 
work where uniformity and freedom from oxidation and dis- 
tortion are necessary. However, where uniformity of heating 
is the only consideration, there is no necessity for using a lead 
bath, because a well-designed furnace will give a uniform heat. 
But where it is essential that a bright surface be maintained, a 
lead bath is indispensable. 

Several minor difficulties are encountered in the use of 
lead. Lead has a tendency to adhere to threaded portions or in 
holes of the pieces being treated, and may seriously affect the 
material during the cooling process. On account of its high 
specific gravity, special means must be taken to keep the pieces 
that are being hardened or quenched from floating on the sur- 
face of the lead, because the steel will not stay underneath the 
surface of the lead by its own weight. At high temperatures, 
lead baths give off fumes which must be taken care of, because 
lead fumes are poisonous. Some manufacturers have found it 
necessary to agitate the bath from time to time in order to 
maintain a uniform temperature throughout the container. 

The temperatures at which lead baths are used depend 
entirely on the use to which they are to be put. For temper- 
ing, the lead bath is used at temperatures of 650 F. or greater. 
If it is desirable to use lead for tempering at temperatures 
lower than 650 F., the melting point of the lead can be low- 



Lead Hardening 45 



ered by alloying the lead with tin or some other low melting 
point metal. 

For hardening, temperatures from 1450° F. to 1750 F. 
are used, but ranges around 1750 F. are the exceptions and 
are not to be recommended. Some manufacturers do not con- 
sider the use of lead good practice at temperatures higher than 
1500 F. 

Until recently one of the chief difficulties in the lead 
hardening process was the failure of containers. A container 
is now being sold which stands up remarkably well under the 
severe conditions to which a lead container is subject. This 
container will be discussed in a later chapter. 



CHAPTER V. 

CARBONIZING CONTAINERS 



CHAPTER V 



Carbonizing Containers 

The direct bearing that carbonizing boxes, pots, retorts, 
etc., have on results and on costs cannot easily be overestimated. 
The use of any container which cracks, warps, scales, etc., is 
not economical from two standpoints. First, it means that a 
large surplus supply of containers must be kept on hand at all 
times. Second, it means that the process involving their use is 
inefficient, consuming additional fuel, material, labor, etc. The 
container used also controls, to a great degree, the product pro- 
duced, and to a greater extent determines the capacity of the 
plant and the production costs. 

It is known to every heat treater that the use of cast iron, 
wrought iron or steel for containers, is far from satisfactory. 
Ferrous containers, whether iron or steel, are not suitable for 
use at temperatures at which they oxidize, and since iron or 
steel oxidizes at room temperature, their life must naturally be 
materially shortened at carbonizing furnace temperatures. 

When a ferrous container is to be used for case hardening, 
an experienced heat treater will generally fill it with carboniz- 
ing compound and put it into the furnace for several hours. 
This is for the purpose of impregnating the ferrous container 
with carbon, so that when it is packed with the pieces that are 
to be carbonized, all the carbon evolved will not be absorbed 
by the container, instead of being absorbed by the pieces in the 
container, as it should be. Preparing the container for service 
in this manner means the loss of one complete furnace heat. 
The loss of one furnace heat would be of little importance if it 



50 Driver-Harris Company 

were not for the fact that this very first heat causes the con- 
tainer to oxidize, forming a scale of iron oxide, and that the life 
of such containers is very short. Scale formation on the exposed 
parts of ferrous containers is known as "growth" and occurs 
invariably when a ferrous container is subjected to high tem- 
perature in the atmosphere or in a furnace. This scale, formed 
as just stated, does more than merely cause a growth of the 
container. Just as scale in a boiler tube decreases the efficiency 
of the boiler, so the scale, or iron oxide, formed on a ferrous 
container, decreases the efficiency of the container, because iron 
oxide is more or less refractory and interferes with heat pene- 
tration. In other words, a portion of the fuel used in the 
furnace is consumed in driving the heat through the scale, 
thereby also lengthening the time element without any beneficial 
result. If the scale which was formed remained in position and 
protected the ferrous container from further oxidation, the heat 
treater would learn to judge his time of heats quite easily. But 
an iron oxide scale is not adhering, nor is it protecting. The 
scaling or oxidizing does not stop, once the scale has formed, 
but continues to grow heavier until, of its own weight and due 
to the difference in expansion under temperature between it and 
the container, it drops off, exposing fresh surfaces of the con- 
tainer to the oxidizing influences. In this way the container 
loses weight, grows materially thinner and lighter, and makes it 
increasingly difficult for the heat treater to produce perfectly 
carbonized pieces. The container finally becomes so thin or so 
light, that it is not safe to use it again, due to the danger of 
burning or overcarbonizing the work contained in it, or because 
it is so frail that it will not support its normal number and 
weight of pieces to be carbonized. Besides the foregoing serious 
defects in ferrous containers, there is also the fact that they 
crack very easily. Cracks, of course, allow the carbonizing 
gases to seep through and in this way the parts being heat 



Carbonizing Containers 51 

treated are frequently damaged and sometimes rendered entirely 
useless. 

It is a custom, when iron or steel containers crack, to lute 
the cracks with clay, but this is only an expedient and does not 
remedy the defects. Ferrous containers, then, may be said to 
fail in almost every imaginable manner. Experience has shown 
that this is equally true of alloy-steels, as well as of specially 
designed and cast containers that are made of special grades of 
crucible or electric steels. It can therefore be stated conclu- 
sively that all ferrous containers, whether iron, steel, crucible 
steel, electric steel or alloy steel, are subject to scaling and will 
scale in carbonizing temperatures to such an extent that they 
are far from efficient. The field has been flooded with these 
so-called heat resisting steels, and failure to get the desired and 
much-needed relief has made heat treaters wary of special steel 
and alloy-steel containers. The average life of all such con- 
tainers is less than two hundred and fifty hours at carbonizing 
temperatures. 

Because of these conditions, the spoilage of expensive ma- 
terials and the high heat treating cost incident to oxidizing and 
cracking containers, have been regarded as unavoidable in many 
heat treating plants. But this was before the alloy known as 
"Nichrome" was developed and cast into Carbonizing and other 
heat treating containers. Five years of practical application and 
use in connection with the most exacting requirements and 
under the most severe conditions, have proved that Cast Ni- 
chrome Containers have solved the problem from every stand- 
point. Records show that Cast Nichrome Containers give a 
service of thousands of hours at 1800 degrees Fahrenheit with- 
out warping, cracking, scaling or other changes of physical char- 
acteristics that interfere with heat penetration. In some cases 
Cast Nichrome Containers have been in active service for 11,000 
hours. 



52 Driver-Harris Company 

The question as to what this "Nichrome" Alloy is and 
what characteristics make it succeed where other containers fail, 
will naturally come into your mind. Therefore, before going 
into the specific advantages and into the uses to which the 
various types of Cast Nichrome Containers are put, the next 
chapter gives some commercial and technical data, as well as a 
little of the history of ''Nichrome." 



CHAPTER VI. 

NICHROME— COMMERCIAL AND TECHNICAL DATA 
Tensile Tests at 70°F. Tensile Tests at 1500 F. 




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CHAPTER VI 

Nichrome 
Commercial and Technical Data 

Alloys of nickel and chromium have been known and used 
both abroad and in the United States for many years as resist- 
ance materials for electrical purposes in the form of wire. The 
many valuable properties of these alloys are known commer- 
cially and technically ; detailed information can be found in text 
books and in a Bulletin published by the Driver-Harris Com- 
pany of Harrison, New Jersey. Therefore, a brief summary 
of these properties will suffice here. Resistance to oxidation at 
extremely high temperatures, uniformity of electrical resistance 
and conductivity, high melting point, and variety of application, 
have made Nickel-Chromium Alloys invaluable for electrical 
purposes. No other resistance material has supplanted them in 
their particular field. 

A number of years ago, the research department of the 
Driver-Harris Company conceived the idea that "Nichrome," 
by reason of its resistance to corrosion at high temperatures, 
might have other valuable industrial applications as a high 
temperature resisting alloy. Much time and effort was spent in 
determining these applications and in working out the material 
to meet them. It was early seen that the demand for such an 
alloy in the heat treating field was great, so that all efforts were 
made to produce cast containers to meet the severe require- 
ments of this particular field. 

Practical 'foundry men and metallurgists had asserted that 
alloys of nickel and chromium could not be successfully cast in 



56 Driver-Harris Company 

sand. Not satisfied with such results as had been obtained by 
others, the Driver-Harris Company proved for themselves that 
the alloy could not be sand cast by ordinary foundry methods. 
It then became necessary, unless the project was to be aban- 
doned, to devise special foundry practices to take care of the 
special qualities of the material which was to be cast. For a 
long time results were indifferent, but as more and more was 
learned about the alloy and its adaptability, results grew pro- 
portionately more encouraging. After many months of con- 
tinued effort, a process was evolved by means of which it was 
possible to successfully sand cast the alloy "Nichrome." The 
demand for these castings became at once established, and their 
use has been limited only by the facilities for their manufacture. 

All the castings made of this nickel-chromium alloy and 
known as Cast Nichrome are fully protected by patents, owned 
and controlled by the Driver-Harris Company of Harrison, 
New Jersey. 

Cast Nichrome can be machined readily with any good high 
speed cutting steel; but due to the toughness of the alloy, the 
tools must be ground with special angles. These particular 
angles have been worked out, and blue prints showing how tools 
should be ground for machining and threading Cast Nichrome 
can be secured on request from the Driver-Harris Company. 

"Nichrome" is a high temperature resisting alloy, being 
practical for use at all temperatures up to 2000 F. At the 
high temperatures encountered in commercial practice, "Ni- 
chrome" resists oxidation to an extent not possible in any other 
alloy, steel or iron. Aside from the fact that it is a high tem- 
perature resisting alloy, as well as an oxidation resisting alloy, 
it may be mentioned that it is practically non-warping, retain- 
ing its shape almost perfectly at temperatures up to 2000 F. 
When the castings are properly designed, there should be abso- 
lutely no warping or cracking. 



NlCHROME 57 



"Nichrome" can be forged at the proper temperature, but 
must be handled very carefully. It is malleable and ductile, but 
during the rolling and drawing operations repeated annealings 
are necessary. 

"Nichrome" can be readily welded by experienced welders 
by either the oxy-acetylene or the electric arc method. The 
nature of the alloy, however, calls for welders of considerable 
skill, as a weld on Cast Nichrome cannot be made in the same 
manner as a weld on cast iron or steel. Pre-heating is almost 
invariably necessary, and thorough cleaning of the surface to be 
welded is important. Best results in welding Cast Nichrome 
are obtained by chipping, with an air or cold chisel, the sur- 
faces of the parts to be welded, so that new, virgin metal is 
exposed. Welding may be done with or without a flux, de- 
pending on whether the weld is a gas weld or an arc weld. If 
oxy-acetylene is used, then a flux is necessary for good results. 
If the electric arc is used, a flux is not needed. At the works 
of the Driver-Harris Company all welding on large, heavy cast- 
ings is done with the electric arc. It is particularly important 
that a too prolonged local over-heating does not take place in 
the welding process as there is grave danger that the weld and 
the parts around it will become porous and spongy. It is for 
this reason that thorough pre-heating is necessary. 

Cast Nichrome cannot be cut successfully with the oxy- 
acetylene cutting torch due to the fact that the alloy does not 
oxidize readily. Since the action of the cutting torch depends 
on its oxidizing action, it can be easily understood why Cast 
Nichrome cannot be cut by this means. A good hack-saw or 
a cold saw is a much cheaper means of cutting the alloy. 

The critical point of "Nichrome" is below room tempera- 
ture and it will not absorb carbon readily until temperatures 
well over 2000 F. have been reached. Besides its great resist- 
ance to oxidation at high temperatures, "Nichrome" is resistant 



58 



Driver-Harris Company 



to the action of most acids and alkalis, hot or cold, and 
"Nichrome" castings are giving very satisfactory service in a 
great many applications in conjunction with the use of acids. 
The curves given below show the resistance of "Nichrome" 
to the ordinary acids. 



THE EFFECT OF VARIOUS CONCENTRATIONS OF AClOi AT 7b°F ON 

NICHPOME CASTINGS 



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PEftCENTA&E OFACTUAL CONCCMTRATION OF ACIDS. 



NlCHROME 59 



The following technical data on Cast Nichrome is given 
in the hope that it will prove valuable to engineers and tech- 
nical investigators: 

CAST NICHROME 

Melting Point 2660 F. 

Softening Temperature 2300 — 2400 F. 

Safe Working Temperature 2000 F. 

Specific Gravity 8.15 

Weight per cubic inch 29 lbs. 

Specific Heat 1 1 1 at ioo° C. 

Brinell hardness with 1000 Kilo weight. . 160 — 170 
Brinell hardness with 3000 Kilo weight. . 179 

Sclerescope hardness 27 to 30 

Coefficient of Linear Expansion over tem- 
perature ranges: 

From o to ioo° C .0000121 per ° C. 

From 32 to 21 2° F 0000091 per ° F. 

Thermal conductivity is 0.0341 calories per centimeter per 
second or in a ratio of 1 to 4.88 of soft iron. Expressed in per- 
centage 20.5% of the thermal conductivity of soft iron. Tests 
made on a 1" diameter specimen. 

Tensile Tests at 70° F. 

Elastic Limit 40,000 lbs. per sq. in. 

Ultimate Tensile Strength 54,ooo lbs. per sq. in. 

Per Cent. Elongation 1 % 

Per Cent. Reduction of Area 2^% 

Tensile Tests at 1500° F. 

Elastic Limit 20,100 lbs. per sq. in. 

Ultimate Tensile Strength 24,500 lbs. per sq. in. 

Per Cent. Elongation 4% 

Per Cent. Reduction of Area 4-3% 



6o 



Driver-Harris Company 



TABLE I 


Tempera- 


Tensile > 


strength in Pounds per Square Inch 


ture 


Cast Iron 


Wrought 
Iron 


Mild Steel 


Cast 
Nichrome 


70° F. 


18,000 


42,000 


55,000 


54,000 


500° F. 


18,000 


48,000 


70,000 




1000° F. 






15,300 




1100° F. 


10,400 








1500° F. 


7,600 


6,300 


9,900 


24,500 


1800° F. 








15,000 


2000° F. 








12,000 


TABLE II 


Tempera- 


Percentage 


Change of Normal Strength at 70° F. 


ture 


Cast Iron 


Wrought 
Iron 


Mild Steel 


Cast 
Nichrome 


70° F. 


Normal 


Normal 


Normal 


Normal 


500° F. 


None 


11^% Inc. 


27% Inc.f 




1000° F. 






72% Dec* 




1100° F. 


42% Dec * 








1500° F. 


58% Dec* 


85% Dec* 


82% Dec* 


55% Dec* 


1800° F. 








72% Dec* 


2000° F. 








78% Dec* 



Note — * Dec means decrease, f Inc. means increase. 

While the results shown in the two preceding tables are 
subject to some changes (due to the varying compositions of 
cast iron, wrought iron, and steel), the figures given show con- 
clusively the superior tensile properties of Cast Nichrome at 
high temperatures. It is also clearly evident that "Nichrome" 
is a much better engineering material for high temperatures than 
iron or steel, because of the fact that its tensile strength at 
i8oo° F. is as great as the tensile strength of mild steel at 
iooo F., and almost as great as the tensile strength of cast 
iron at 70 F. 



NlCHROME 



Cast Nichrome will bend considerably without breaking 
at either a red or a white heat. The metal does not become 
easily fatigued. This fact can be readily demonstrated by sup- 
porting a plate of Cast Nichrome Y\" thick at the four cor- 
ners, and striking the unsupported center with a heavy sledge. 



CHAPTER VII. 

CAST NICHROME CONTAINERS 

Cast Nichrome for Cyanide Hardening — Cast Nichrome for 
Lead Hardening — Cast Nichrome for Pyrometer Protection 
Tubes — Dipping Baskets — Additional Uses for Nichrome 
Castings. 



CHAPTER VII 



Cast Nichrome Containers 

In a preceding chapter it was stated very emphatically that 
"Nichrome" did not scale at temperatures well above the range 
of case carbonizing processes. The fact that "Nichrome" does 
not scale was learned in early experiments, and was the pri- 
mary cause of the development of Cast Nichrome for carboniz- 
ing containers. Since no scale is formed at carbonizing tem- 
peratures, the walls of Cast Nichrome containers can be made 
thin and the container itself will then be correspondingly light 
in weight, while leaving to the heat treater the assurance that 
his container will not go to pieces in the furnace. Not forming 
a scale, Cast Nichrome containers require no more temperature 
to produce equivalent results, in subsequent heats, than was re- 
quired for the first heat. Not forming a scale, the weight of a 
Cast Nichrome container is constant, so that a heat treater is 
assured of comparable results from every heat. 

The life of Cast Nichrome containers is exceptionally long, 
being from thirty to fifty times as long as the life of any fer- 
rous container under the most severe furnace conditions. It is 
not necessary to maintain a reducing atmosphere in the furnace 
to secure long life from Cast Nichrome containers. On the 
contrary, a slightly oxidizing atmosphere is preferable. 

Users of Cast Nichrome carbonizing containers can attest 
to the fact that the life of these containers at a temperature of 
1800 F. runs into thousands of hours. Records are on file in 
the offices of the Driver-Harris Company proving that one 
large user of Cast Nichrome Containers has secured as much as 



Cast Nichrome Containers 67 

eleven thousand hours from some of his containers. The average 
life generally is between five thousand and seventy-five hundred 
hours' service under average conditions. 

The fact that Cast Nichrome Containers are tougher and 
stronger at elevated temperatures allows thinner castings to be 
made of this material. As a consequence the heat travels 
through the walls of the container much more quickly and con- 
siderable fuel is saved. 

One of the largest case-carbonizing plants in the middle 
West, using low sulphur producer gas for fuel, has shown by 
accurately kept log sheets, that by the substitution of Cast 
Nichrome Containers for steel containers the fuel consumption 
per furnace was reduced from 420 cubic feet per hour to 265 
cubic feet per hour, effecting a saving of 155 cubic feet of pro- 
ducer gas each hour of operation per furnace. Because of the 
rapidity of the heat penetration through Cast Nichrome Con- 
tainers, the length of heats for this concern was reduced from 
nine hours to six and one-half hours, thus effecting a saving of 
two and one-half hours for each furnace per heat. These figures 
are authentic, and the name of the company from which they 
were secured will be given on request. 

All heat treaters know the trouble experienced from the 
cracking and warping of ferrous containers. Cast Nichrome 
does not crack or warp, so that the heat treater knows posi- 
tively that he will always be able to put the same number of 
containers in the furnace heat after heat, and is further assured 
that the covers for the containers will fit throughout the life of 
the containers. 

Since Cast Nichrome Containers do not scale, crack, or 
warp, they can be made in designs not feasible with ferrous con- 
tainers. Cast Nichrome rectangular pack hardening boxes are 
made with a cover which fits the box much as a shoe box lid fits 
the box itself. This container, commonly known as the "shoe 




Fies 2 2A. Cast Nichrome Open-Chimney Carbonizing Box with 
test hole 'and cover. (Bottom)— Cast Nichrome Carbonizing Box with 
lifting lugs and runners instead of legs. 





■^ 



Fig. 3. Cast Nichr 



ome Carbonizing Pots with "shoe box cover 
and lifting lugs. 




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a, 

"3 
J8 



s 



Cast Nichrome Containers 71 

box cover" design, is sold to the user with the assurance that the 
cover can be replaced on the box, after any number of heats, as 
easily as when the container was delivered new. Also, owing to 
this particular design, the efficiency of the container is greatly 
increased in that a positive pressure, so beneficial to carbonizing, 
is always maintained. Less fire clay luting is required (some 
users have discontinued the use of fire clay with this container), 
and there is no ashing of the carbonizing materials. Due to 
the toughness of the alloy itself, no particular care is necessary 
in handling the containers. 

The "shoe box cover" container is shown in Fig. 1, page 
66. A detailed, mechanical sketch is shown in the Appendix, 
together with a list of the standard sizes of this container, the 
patterns for which are kept in stock by the Driver-Harris Com- 
pany, Harrison, New Jersey. 

For many classes of work to be case-carbonized, heat 
treaters have found the rectangular box unsuitable. Therefore, 
the Driver-Harris Company has designed a round Cast 
Nichrome Container made with the "shoe box cover," and from 
which the same excellent results can be obtained. This round 
box is shown in Fig. 3, page 69. The sizes of this Cast 
Nichrome Container, for which patterns are kept in stock by 
the Driver-Harris Company, are given in the Appendix, to- 
gether with a mechanical sketch of the container. 

The fact that Cast Nichrome serves for thousands of hours 
at 1800 F. without warping, cracking or scaling has led to 
the invention and patenting of a Cast Nichrome Container 
which is unique in the heat treating and case carbonizing in- 
dustry. This Cast Nichrome Container is a round container 
with a machined, taper fit cover, which fits tightly into the con- 
tainer and effectually closes the container, much as a ground 
glass stoppered bottle can be tightly closed by a simple turn of 




u 



■OJD 



Cast Nichrome Containers 73 

the stopper. Such a container would not be possible with any 
other material but "Nichrome." This fact is self-evident be- 
cause a container that scales or warps could not possibly be 
expected to maintain a tapered seal after the first heat. Cast 
Nichrome does not scale or warp, so that the machined cover fits 
the taper of the container as well after several thousand furnace 
hours as when the container is first used. 

This patented container is known as the "Sealtite" pot, and 
is used by many of the largest case-carbonizing plants in the 
United States. There is absolutely no luting required with this 
design pot, the seal formed by the seating of the machined cover 
in the machined container being tight enough for any carboniz- 
ing operation. In addition to its longer life this Cast Nichrome 
"Sealtite" container is from fifteen to twenty per cent, more 
efficient than any ferrous container. Pressure, due to liberated 
carbon gases, in this type of container, may be developed to 
such an extent as to blow the cover off the pot. This has been 
the experience of one large user, who blew off the covers many 
times before he became convinced that the gases were being 
generated in his pots faster than they could be absorbed. When 
he was finally convinced that a great saving in compound was 
possible by the use of this Cast Nichrome "Sealtite" Container, 
he was able to save some fifteen per cent, of his fuel, cut his 
time by one-quarter, and do away entirely with any luting of 
the pots. 

This "Sealtite" Container is shown in Fig. 4, page 70, and 
stock pattern sizes, together with a mechanical sketch of the 
container, are shown in the Appendix. This type of container 
is also made in the "chimney-pot" design for the carbonization 
of ring gears, large ball races, etc. 

Annealing tubes and carbonizing tubes of Cast Nichrome 
are made with one end closed and with the other end threaded, 
so that a "Nichrome" cap can be screwed on, thus effectually 




u 



Cast Nichrome Containers 75 



closing the tube. These tubes are used in many large indus- 
trial processes, such as the ball-bearing industry, the cam shaft 
industry, and in the treatment of wire for incandescent lamp 
manufacture. Many standard patterns for these tubes, pic- 
tured in Fig. 5, page 72, are kept in stock by the Driver- 
Harris Company. 

It was stated in the chapter devoted to Gas Hardening, 
that the principal part of the Gas Hardening Machine is a ro- 
tating retort. Until the introduction of Cast Nichrome Re- 
torts in the industries, these rotating retorts were always made 
of a ferrous material giving a life of from three to four weeks 
at the temperatures necessary for carbonization. Since the in- 
troduction of Cast Nichrome Retorts, users have been able to 
secure a life of from eighteen months to two years from each 
retort at temperatures of 1700 F. to 1800 F. The saving 
effected in labor, furnace repairs, fuel, production, time, etc., by 
these Cast Nichrome Retorts, has led to their adoption as stan- 
dard equipment by one of the largest gas hardening furnace 
manufacturers in the United States. These retorts can be 
made in any size and weight up to five thousand pounds. More 
than sixty retorts weighing twenty-five hundred pounds each 
were supplied to the Gas Defense Division of the United States 
Army during the recent World War, and were operated at a 
temperature of 2000 F. in the production of Gas Mask Car- 
bon. When it is further understood that these Cast Nichrome 
Retorts operated in an atmosphere of superheated steam, the 
remarkable non-oxidizing qualities of the alloy will be appre- 
ciated. Cast Nichrome rotary retorts are effecting great sav- 
ings for some of the largest steel ball manufacturers. 

They are made to specifications either as a straight cylin- 
drical retort, or as a spiral retort for continuous furnace 
operation. 







Cyanide Furnace with Carry-off flue. 



Cast Nichrome for Cyanide Hardening 77 

Cast Nichrome for Cyanide Hardening: 

Users of liquid carbonizing containers, such as cyanide 
pots, are well aware of the difficulty encountered in securing a 
satisfactory ferrous container. The use of Cast Nichrome con- 
tainers effectually solves this problem and assures the heat 
treater thousands of hours of continuous service, without the 
exasperating delays due to the container springing a leak and 
necessitating its removal from the furnace. Where the furnace 
life of a ferrous container may be from 50 to 150 hours, no ex- 
traordinary skill is necessary to secure from fifteen to twenty 
times this service from a Cast Nichrome container. At the 
temperatures used for cyanide hardening, Cast Nichrome is as 
resistant to the action of the molten cyanide, as it is to oxidiz- 
ing influences at temperatures of pack-hardening. 

The Research Department of the Driver-Harris Company 
has found that the most efficient and economical cyanide fur- 
nace is one in which the cyanide pot is sealed into the furnace 
by means of a seal of chrome ore or magnesite. The seal is 
placed between the brick lining and the flange of the pot. In 
this type of furnace the waste gases of combustion are led into 
a separate flue, and there is, therefore, no connection between 
the combustion chamber and the hood which covers the cyanide 
pot. This method of construction effectually prevents the 
cyanide vapors from passing into the combustion chamber. Since 
it has been proven that cyanide in the combustion chamber 
greatly reduces the life of a pot, the life of Cast Nichrome 
Cyanide Pots, under conditions just outlined, should be long 
enough to show marked economy. 

Besides the great lengthening of pot life by this furnace 
construction, there are the added advantages of a large saving 
of cyanide, an increase in the life of the furnace lining, and a 
more even temperature. 



Cast Nichrome for Lead Hardening 79 



Cast Nichrome for Lead Hardening: 

Heat treaters, who prefer to harden in molten lead, have 
experienced the same difficulty with ferrous containers as have 
the cyanide hardening users. Cast Nichrome containers solve 
the lead container problem effectually and assure the heat 
treater many thousands of hours' life at the average tempera- 
ture of lead hardening. Round, rectangular, and bath tub 
shaped lead pots are made of Cast Nichrome in all sizes and 
weights, from a small pot of five or ten pounds for cutlery 
hardening, to a large Cast Nichrome bath tub of two thousand 
pounds for the automobile industry. Covered Cast Nichrome 
rectangular lead pots, for continuous wire-tempering furnaces, 
are used by the largest wire manufacturers in this country. 
Figs. 7, 8, 9 and 10 show Cast Nichrome cyanide and lead 
containers, and stock sizes will be found in the Appendix. 

Cast Nichrome for Pyrometer Protection Tubes: 

Cast Nichrome pyrometer protection tubes need no intro- 
duction to American industry. For some years they have been 
standard equipment of the leading pyrometer manufacturers. 
Their superiority to ferrous tubes is so well established that no 
comparison is needed. From one to two years' continuous serv- 
ice under the most severe conditions of temperature and fuel, 
have established Cast Nichrome pyrometer protection tubes in 
the plants of the world's manufacturers. 

They are made in many designs and innumerable sizes, 
and can be supplied plain, threaded, flanged, light walled, or 
heavy walled, dependent on the conditions for which they are 
supplied. 

Cast Nichrome pyrometer protection tubes are shown in 
Fig. 11, page 83, and stock pattern sizes are listed in the 
Appendix. 





Fig. 8. Cast Nichrome Lead or Cyanide Pots. 



Il 



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Fig. 9. Cast Nichrome Lead Bath Container — Tub Design. 




u 



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Ss 



p. s 




«Svl 












: 

I 



v; 








Fig. 12. Cast Nichrome 



Dipping Baskets for cyanide hardening. 



Dipping Baskets 85 



Dipping Baskets: 

One of the most important industrial applications of Cast 
Nichrome is the "dipping basket" used for the purpose of im- 
mersing small parts in a molten cyanide bath or for immers- 
ing small parts in heat treating quenching baths or pick- 
ling baths. Its life under these rigorous conditions is practi- 
cally endless, as it shows no tendency to grow, shrink, warp, or 
crack. Many large automotive manufacturers have entirely re- 
placed their ferrous basket equipment with Cast Nichrome bas- 
kets. Fig. 12 shows the types of Cast Nichrome dipping basket 
most commonly used. 

Additional Uses for Nichrome Castings: 

Cast Nichrome has a wide range of other industrial uses, 
such as glass molds for the glass bottle industry ; molds for the 
die casting industry; conveyor baskets and chains for continu- 
ous furnaces of many kinds; furnace parts; muffles for ore- 
roasting, etc. 

The Driver-Harris Company maintains an engineering 
department to which inquiries as to the adaptability of Cast 
Nichrome for industrial purposes other than those which are 
ordinary fields, can always be referred. 



CHAPTER VIII. 

COMMERCIAL METHODS OF USING CAST NICHROME 
Automobile Starting and Lighting Equipment— Studs— Set 
Screws— Small Bolts— Nuts— Screws— Etc.— Ring Gears for 
Automobile Differentials— Roller Bearings. 




bJO 

a 

'$ 

o 

CO 



CHAPTER VIII 

Commercial Methods Of Using 
Cast Nichrome 

It has been stated elsewhere in this book that the use of 
Cast Nichrome containers offers many advantages to the heat 
treater, among them being a saving of fuel, and therefore of 
furnace repairs ; a saving in time of operation ; a saving in labor 
costs; a saving in production costs; greater production from 
the same furnace equipment ; and a greater uniformity in prod- 
uct. Since these advantages are so marked, and since data is 
available from so many large manufacturers using Cast Ni- 
chrome containers, a brief outline of carbonizing methods used 
in representative plants of the several industries doing carbon- 
izing is given here in the belief that these methods will effect 
similar savings for others who may not be using Cast Nichrome 
containers. 

Automobile Starting and Lighting Equipment: 

Under this head are included small shafts, cams, ball races, 
sprockets, etc. The pieces to be carbonized are packed in Cast 
Nichrome carbonizing boxes of the "shoe box cover" or "Seal- 
tite" design. Both styles of containers can be advantageously 
used for this class of work. The size of the container should 
be determined by the size of the furnace and by methods avail- 
able for handling. No fire clay luting is necessary. Hydro- 
carbonated bone black, hydrocarbonated lamp black or any 
other good carbonizing compound can be used. The contain- 
ers, packed with the pieces to be carbonized, are placed in the 



90 Driver-Harris Company 

furnace and subjected to a temperature of 1700 F. for six and 
one-half to seven hours, if the parts to be case-carbonized are 
made of .20 carbon steel. If they are made of a 3.5% Nickel- 
steel, the temperature need not exceed 1650 F. At no time is 
it necessary to run temperature over 1700 F. for this small 
work. 

After the case-carbonizing operation is completed, the 
boxes are drawn from the furnace, covers removed, and con- 
tents allowed to cool. Then the carbonizing material is riddled 
out. The pieces which have been carbonized are then reheated 
to 1 475 F. and drawn at 400 F. If shafts of a carbon con- 
tent of 40% have been carbonized, the reheating temperature 
should go as high as 1550 F. and the drawing should take 
place at 350 F. 

Studs, Set Screws, Small Bolts, Nuts, Screws, Etc.: 

Material of this nature is carbonized in a cyanide bath in 
a Cast Nichrome container, and immersed in the cyanide bath 
by means of the Cast Nichrome dipping basket. For best re- 
sults, a bath of cyanide made up of equal parts of salt, soda ash, 
and sodium cyanide is excellent. The temperature need not be 
over 1500 F. and the time of immersion should be about 
twelve minutes. After the carbonizing operation, the material 
should be quenched as quickly as possible in oil or water. 

Ring Gears for Automobile Differentials: 

Gears of this type are effectually carbonized in Cast Ni- 
chrome Sealtite Chimney Pots. The compound used for car- 
bonizing is made up into a paste and mechanically pressed into 
the teeth of the gear. The gears are stacked one on the other 
and placed carefully in the Cast Nichrome container. Tem- 
perature of carbonization is 1 700° F. and 3/32" case is obtained 
in eight or nine hours. 



Commercial Methods 9£ 



Small pinions which cannot be stacked conveniently are 
carbonized in Cast Nichrome closed end tubes in which the 
pinions are placed and packed with a loose compound. They 
are carbonized at a temperature of 1700 F. in eight to nine 
hours. 

After the carbonization is complete, the gears are taken to 
a reheating furnace without being allowed to cool. The re- 
heating should be done at 1500 F. and the time of treatment 
should be one hour. When a temperature of 1500 F. has 
been maintained for one hour, the gears are quenched in oil. 
In order to make certain that both the case and the cover have 
been refined, a second reheating to a temperature of 1400 F. 
should be given the gears, and a second oil quenching should 
follow. 

Another method of treatment of ring gears is to carbonize 
in Cast Nichrome "Sealtite" pots with loose compound at a 
temperature of 1600 to 1650 F. for ten hours. The gears 
are allowed to cool in the pots. When cool they are thor- 
oughly cleaned and the subsequent reheatings for heat treat- 
ment is done in molten lead contained in Cast Nichrome lead 
pots. 

It is possible to quench differential gears directly from 
the carbonizing box, and then give one subsequent reheating in 
lead. In this way, one reheating is done away with. 

Roller Bearings: 

Roller bearings are carbonized in Cast Nichrome rectan- 
gular carbonizing boxes, and the cups and covers are similarly 
carbonized in Cast Nichrome ''Sealtite" Pots with a gas seal 
cover The length of heat is fourteen hours, and the tempera- 
ture is 1600 to 1650 F. The long heat is made necessary by 
the depth of case which is desired. The rollers of the roller 



92 Driver-Harris Company 

bearing, after carbonizing, are cooled in the boxes and then 
reheated in large Cast Nichrome rotary retorts, the operation 
being continuous. The rollers, after passing through the re- 
tort, are discharged into an automatic quenching device. 



APPENDIX. 

STOCK PATTERNS AND SPECIAL CONTAINERS 

The widely varying requirements and operating condi- 
tions in case carbonizing and other heat-treating processes 
call, of course, for engineering practice in applying the proper 
containers. The Driver-Harris Engineering and Research 
Organizations are constantly co-operating with the men re- 
sponsible for results in the heat-treating departments of the 
various industries. 

Such service naturally brings with it a broad experience, 
as well as a number of standardized designs which have proved 
their efficiency and which can be used in many heat-treating 
plants. 

The tables on the following pages show some of the stock 
patterns and sizes in which Cast Nichrome Containers are 
made. 



O 
PQ 

O 

z 

N 
Z 

o 

U 



Pat- 
tern 

No. 


CO 

G 

i— i 


Weight 
of Lid- 
Esti- 
mated 


Weight 
of Box- 
Esti- 
mated 


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co 

co 

.s 

co 

C 

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'co 

d 

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HS ^ ^ ^ HS ^\ ^ H2 ^ ^ ^ HS ^\ 



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